Nuclear reactors, because of the requirement for absolute safety in operation, generally are provided with a number of independently triggerable or operable control or regulating systems for the reactivity of the reactor core and, in an emergency or for other reasons, for shutting down the nuclear reactor. In general at least two safety systems are provided of such nature that, once the emergency has abated, the reactor can once again be placed in operation.
In German open application No. DE-OS 27 53 928 and the corresponding U.S. Pat. No. 4,279,697, a safety system for shutting down a nuclear reactor has been described in which a gadolinium-containing substance is introduced into the cooling gas circulation of a stacked or piled ball reactor.
Such a reactor generally comprises a column through which the fuel elements, in the form of graphite balls containing the fissionable material are circulated, usually with a residence time corresponding to the fissionable life of the material of this element. The fuel balls, therefore, generally pass through the column or stack only once. The cooling gas is circulated directly through the column and hence through the interstices of the stack.
When the gadolinium-containing substance is introduced into the gas circulation, gadolinium is deposited on the porous surfaces of the graphite fuel balls of the reactor core on the surfaces of the open pores of the graphite.
The more gadolinium which is deposited upon the surface, the lower will be the reactivity of the nuclear reactor because of the resulting absorption of neutrons.
Since gadolinium in its natural isotopic mixture has practically the highest absorption cross section for thermal neutrons of naturally occurring chemical elements, for shutdown of a nuclear reactor of the piled ball type, it suffices to deposit a comparatively small amount of gadolinium on the surfaces of the nuclear fuel elements.
Since the gadolinium is deposited upon the fuel balls which are replaceable, it suffices for reactivating the reactor to desorb the gadolinium from the fuel elements by raising the reactor temperature and flushing it through with gas that does not contain gadolinium, or to decompose the gadolinium by nuclear reaction into other nuclides, or by a combination of these methods. The decomposing of gadolinium can be carried out, if necessary, by adding fresh nuclear fuel elements into the reactor core.
The gadolinium-containing substances which are used are generally gadolinium compounds which can be introduced as sols, solutions or gases into the reactor core. For shutdown of such a reactor an aqueous gadolinium acetate solution has been found to be preferable whereas the gaseous gadolinium compounds which are preferred are gadolinium aluminum isopropoxide Gd(Al(C.sub.3 H.sub.7 O).sub.4).sub.3 and gadolinium tricyclopentadienyl Gd(C.sub.5 H.sub.5).sub.3, the latter, having no liquid phase, being introduced as a vapor of a solid upon sublimation.
When a bypass to the cooling gas circulation is used to feed the substances into the reactor, one must be certain that the cooling gas circulation is intact in spite of the development of the emergency condition which necessitated the quenching of the reaction. Naturally, should the emergency result from a breakdown of the gas circulation, this mode of emission of the quenching compounds will not be satisfactory. Accordingly, this earlier system can require the provision of additional piping systems for admission of the solution of gas containing the quenching agents.